NAUI Nitrox Diver
of 1.3 as their personal maximum. After selecting the
PO2, move down the column to the row that shows the
planned depth, rounding up if the exact depth is not on
the table. The intersection of column and row shows the
best mix oxygen percentage for the dive.
CALCULATING BEST MIX
Calculating best mix is similar to the calculation
for maximum operating depth in reverse.
First, convert the target depth into the absolute
pressure. Next, determine the fraction of oxygen in the
mixture that will produce the maximum acceptable
PO2 at that absolute pressure. This can be done in two
separate steps, or the two steps can be blended into a
single formula.
Example: What is the best mix for a dive to 30 msw
(99 fsw) if the oxygen partial pressure is not to exceed
1.4 atmospheres?
Using the U.S./Imperial depth (99 fsw):
Step 1: Calculate the absolute pressure at 99 fsw. Divide
the depth by 33 fsw/atm to determine the hydrostatic
pressure; then add one atmosphere for the air pressure at
the surface to find the absolute pressure. Use either of
the formulas in Chapter 2:
Pata D fsw 1
= + or
and enter the known values:
Pata D fsw fsw
= 4 ata
= + 33
Pata = 99 fsw +
1
Step 2: Find what fraction of oxygen will produce the
target PO2 at 4 bars/atmospheres absolute. The basic
formula (the part is a fraction of the whole) is:
Pg = Fg x Ptotal
Or, since you know the target partial pressure and the
absolute pressure:
Fg = Pg / Ptotal or FO2 = PO2 limit / P total
Enter the known values:
FO2 = 1.4 ata / 4 ata = 0.35
EAN35 is the best mix for the dive. The procedure is
identical in an S.I./metric calculation.
The two-step process can be combined into a single
formula by replacing the absolute pressure in Step 2
with its formulaic equivalent from Step 1:
⎛
FO PO D fsw
×
⎞
= ⎟ ⎟
⎠
⎜ ⎜
= ÷ +
2 2 limit 33 /
+
⎝
Or, in the example:
fsw atm
FO atm fsw atm
1
= 1.4 × 33 /
= 0.35
fsw fsw
2 +
99 33
PO 33 fsw /
atm
2 limit
D fsw fsw
USING THE OCEANX TO ESTABLISH
BEST MIX
33
The OCEANx calculator can be used to determine
your best mix. Beginning with the O2% window set at
40%; rotate the upper wheel counter-clockwise while
observing the bottom window area that shows actual
depth. Turn the wheel until you see your target depth
appear next to the value 1.4 (or your chosen maximum
PO2) in the PO2 column. If the exact target depth does
not appear, use the next window that will show the next
greater actual depth adjacent to 1.4. Your best mix for
the dive is now displayed in the O2% window.
When diving with oxygen-enriched air, it is essential
that you know the maximum operating depth for
the gas with which you are diving. The MOD should be
atm
fsw atm
33 /
fsw atm
33 /
atm
fsw atm
33 /
48 Choosing the Best Nitrox Mix
FIGURE 4-4: THE OCEANX CALCULATOR SIMPLIFIES NITROX DIVING